1. Field of the Invention
The present invention relates generally to CDMA (code division multiple access) communication systems and more specifically to an interference cancelling technique for CDMA mobile stations.
2. Description of the Related Art
A multi-user interference canceller is known in the art of CDMA cellular communication systems as a means for combating the problem interference that results from asynchronous transmission of spread codes from mobile stations, as shown and described in Japanese Patent Publication 2000-315993. Being designed for use in a base station, the prior art multi-user interference canceller comprises a plurality of interference cancellers connected in successive stages, each interference canceller including a plurality of canceller units or interference replica estimators connected in parallel to simultaneously receive an incoming CDMA signal. The interference replica estimators are provided in number corresponding to the maximum number of data channels the base station can assign to mobile stations. In each stage, the interference replicas are summed in an adder and then subtracted from a delayed CDMA signal to produce an interference-cancelled residual signal, so that interference components of all users are cancelled from the received signal. In each stage, the residual signal is summed with the interference replicas of the individual data channels to produce symbol replicas and supplied to the next stage. The quality of the data channels is improved as the signals proceed from one stage to the next A number of decoders are connected to the outputs of the final stage to provide decoded symbol sequences.
In the case of mobile stations, the quality of communication link is guaranteed by assigning each mobile station a number of data channels to provide code diversity. Depending on the traffic, the number of assigned channels (or spread codes) is varied. In the HSDPA of W-CDMA, a maximum number of fifteen data channels is assigned to a mobile station if no other mobile users exist in the same coverage area of a base station. If there are three mobile users in a cell, they are each assigned a set of five spread codes, for example. In this case, each mobile user has no way of knowing which spread codes are assigned to the other mobile users. Therefore, in each mobile station, interference is cancelled only with respect to the assigned data channels. The signals spread with the unknown spread codes would be combined to produce an interference on each mobile user.